High-Valent Ni Species Induced by Inactive MoO₂ for Efficient Urea Oxidation Reaction

The microenvironment tuning of Ni species, a promising non-precious catalyst, is significant in the energy and environmentally relevant urea electro-oxidation reaction (UOR). Herein, we found that the high-valent Ni species induced by the inactive MoO₂ in mixed nanocrystals of NiO/MoO₂ were effective for urea oxidation. The redox interaction of MoO₂ and NiO revealed by the spectroscopic analysis well supported the formation of high-valent Ni species and the changes in the surface chemical state. High catalytic activity and stability for urea oxidation were observed by a series of electrochemical measures compared to the counterpart catalysts of MoO₂ and NiO. The optimal NiO/MoO₂ hybrid catalyst showed a UOR activity of 73.1 mA cm^-2 at 1.50 V, which was about 12-fold that of the NiO catalyst. In addition, largely improved catalytic kinetics and catalytic stability for UOR were also demonstrated. Because of the inactive activity of MoO₂ and the low performance of NiO, the largely improved preference can be affirmatively attributed to the efficient catalytic synergism of NiO/MoO₂ in the mixed nanocrystals. The current finding clarifies the catalytic promotion effect of the inactive Mo species on Ni-based catalysts for urea oxidation, which would be instructive for Ni/Mo-relevant catalyst development.